Harmonic suppression technique based on mode coupling control in microstrip multistage coupled ring bandpass filters

A technique to suppress higher harmonics in coupled ring bandpass filters is proposed and experimentally demonstrated. This technique basically consists of mitigating the coupling mode between the excitation lines and the microstrip ring, which generates the second harmonic ring resonance. This mode suppression is achieved by optimizing the length of an open-end microstrip line connected to the excitation-coupled lines of the filter. A harmonic suppression ratio for the second harmonic greater than 25 dB in a flat bandpass filter centered at 2.5 GHz is experimentally demonstrated.